The increased deployment of Gigabit differential signals in IC components and systems for high-speed telecommunication applications presents challenges in signal analysis and measurement. One such challenge resides in skew alignment of the high-speed differential signals as represented by differential eye diagrams. Today's telecom systems are designed to move large amounts of data at bit rates that far exceed 1 Gbit/sec, with signal edge durations below 200 picoseconds. At these speeds, effects once deemed secondary, such as differential skew, can become prominent.
The timing skew between a pair of differential signals is called differential skew. The differential skew introduced by matching cabling and adapters used to connect test equipment and devices under test (DUTs) can add to the skew by as much as several hundred picoseconds. Considering that the bit period of a 40 Gbps signal is only 25 ps, an accurate auto skew alignment is mandatory for high speed differential signal analysis and measurement.
The traditional eye diagram crossing-point matching or edge-matching method (see for example Derickson, “Fiber Optic Test and Measurement,” Chapter 8, Prentice-Hall, 1998) is subject to both-amplitude noise and timing-jitter. Since this traditional method is based solely on measurement algorithms, it depends strongly on signal type, e.g., Return to Zero (Rz) or Non-Return to Zero (NRZ), (see for example Keiser, “Optical Fiber Communications,” McGraw-Hill, 1983, pp. 230-233), and tends to break down or become unstable when measurement fails.
The accuracy of the synchronization mechanism utilized in BERT (bit error rate test) instruments for aligning two out of sync signals is dependent on the bit period of the signals. The bit-synchronization mechanism commonly adopted in telecommunications likewise suffers from accuracy problems and complex digital logic. In other words, no existing sync or alignment mechanisms address the compelling issue of cable/connector skew compensation for accurate differential signal analysis and measurement.                F1 and F2 are said to be paired differential signals ifF1(t)+F2(t)=C,  (1)        where C is a constant, independent of time t. The signals could be either optical or electrical, although in reality only electrical differential signals are presently used. When there is a differential skew (ts) between the 2 signals, for example, F2=F2(t+ts),        then the differential signal becomesS(t)=F1−F2=F1(t)−F2(t+ts).  (2)        Assuming that ts is comparatively small, S(t) may be represented asS(t)=F1(t)−F2(t)−ts*F2′(t),  (3)        where F2′(t) is the time derivative of F2(t).        Substituting (1) into (3) and considering timing jitter effect:S(t)=2*F1(t)−C−(ts+2*Tjitter)*F2′(t),  (4)where Tjitter is the average jitter, and F′(t) is the time derivative of F(t). As expected, differential skew (ts) aggravates jitter measurement. For 40 Gbps signals, jitter measurement of differential signals can be completely blurred due to the differential skew introduced by the measurement instrument and peripherals. In other words, the presence of ts distorts the output differential signal S(t), particularly in the rising/falling edge part of the waveform when the derivative F2′(t) is not zero. The distortion becomes apparent in eye diagram parameter measurements such as rise and fall time of high-frequency signals.        
FIG. 1 illustrates an example of 40 Gbps NRZ differential eye diagrams at various skew misalignment stages. Measurement results are derived from the subtraction of the input differential signals from Agilent ParBert with matching cables and connectors linked to Agilent Infiniium DCA through 86118A remote head module. A tri-level differential eye diagram due to grossly misaligned skew (>1 bit period) in the input differential signals is illustrated at 101a. A distorted differential eye diagram due to moderately misaligned skew (from about 0.25 to 1.0 bit period) in the input differential signals is illustrated at 101b. A “good” differential eye diagram due to slightly misaligned skew (less than about 0.25 bit period) in the input differential signals is illustrated at 101c. However, there is still about 4 ps skew in the slightly misaligned input differential signals. Even though misalignment at 101c is hardly perceptible visually, even this slight misalignment can affect eye measurement significantly, especially the rise/fall time, where tr is rise time and tf is fall time as illustrated at 101d. In this example, ideal alignment is reached when ts=0, at which point tr, tf are each roughly 7 ps. It is readily observed that for a 40 Gbps differential signal as shown in FIG. 1, only 3-4 ps of skew misalignment can result in rise/fall time measurement errors as much as 1.5 ps, or ˜20% relative error.
Eye diagram measurement is described generally in Hart et al., “Firmware Measurement Algorithms for the HP 83480 Digital Communications Analyzer,” Hewlett-Packard Journal December 1996, Article 1, p. 1; Scott et al., “Removal of Cable and Connector Dispersion in Time-Domain Waveform Measurements on 40 Gb ICs,” jonathanscott@ieee.org; Cai et al., “Digital Serial Communication Device Testing and Its Implications on Automatic Test Equipment Architecture, IEEE ITC International Test Conference 2000, Paper 23.1, p. 600; Agilent Technologies data sheet DCA 86118A, http://www.agilent.com/cm/rdmfg/commanlyz/86118a/index.shtml; and link to general information regarding Agilent Technologies DCA 86100A/B, http://www.agilent.com/cm/rdmfg/commanlyz/85100a/index.shtml.